The present invention relates to a laser beam scanning apparatus which scans a laser beam in one dimension, and, more particularly to a laser beam scanning apparatus which is effective in scanning a plurality of laser beams at the same time.
It has been known that a conventional laser printer scans one laser beam with a rotating polygonal mirror. However, following the recent requirement for high-speed operation and the recent application of a semiconductor laser array, printers for simultaneously scanning a plurality of laser beams have been developed.
Since an array using a semiconductor laser has a light emitting point size from 1 .mu..phi. to 3 .mu..phi., it is necessary that a fine-turned array is prepared and the distance between semiconductor lasers is set at 1 .mu..phi. to 3 .mu..phi. in order to carry out an elaborate beam scanning. Narrowing the distance between semiconductor lasers, however, results in a heated crosstalk, which is not desirable.
A plurality of lasers are used as an alternative method to avoid the above problem. According to this method, two laser beam sources each having a linear polarization characteristic are amalgamated in approximately the same direction, as disclosed in U.S. Pat. No. 4525024, for example. In this patent application, an optical device (a polarization prism) is proposed which passes a specific linearly polarized light beam (P polarized light beam) but reflects a linearly polarized light beam (S polarized light beam) which vibrates in a plane that is orthogonal with this specific linearly polarized light beam. As shown in FIG. 2, a P polarized beam 100P and an S polarized beam 100S are introduced into a polarization prism 100 PBS and the two beams are output in the same direction, so that the two beams can be utilized effectively.
There have been attempts to compose a laser printer by using a beam amalgamating apparatus as described above. For example, there is an example in which laser beams obtained in the same direction are scanned simultaneously so that the power of laser and the number of revolutions of a rotating polygonal mirror are halved (the preparatory paper for the lecture meeting of the Japan Society of Applied Physics in Autumn of 1985, 3P-H-9, pp 63).
FIG. 3 shows an optical apparatus having the above-described structure. Two laser beams 6.sub.P and 6.sub.S emitted from two semiconductor laser 5.sub.1P and 5.sub.1S are set to be orthogonal with each other at their polarized surfaces, and these two laser beams are amalgamated together in the same direction by a polarization prism 3 so that the two laser beams are scanned simultaneously.
In this case, in order to maintain the spot distance obtained from the two lasers in a sub-scanning direction at a predetermined value, parts of the beams are introduced into a beam detector for detecting the spot distance so that the spot distance is controlled. The control of the spot distance is carried out by using a control circuit 16 which drives galvanomirrors 17.sub.1 and 17.sub.2.
In the above-described technique, polarization directions of the amalgamated laser beams are orthogonal with each other. It is known that when laser beams having polarized light beams which are orthogonal with each other are reflected on a metal reflection surface, reflection factors are different depending on the difference of polarization directions. The difference of reflection factors necessarily occurs from the difference of boundary conditions at a boundary surface of electromagnetic waves when the equation of Maxwell is solved, assuming a beam is an electromagnetic wave. For example, the "Principle of Optics" by Jenkins and White (McGraw Hill, the fourth edition, 1976, pp. 535) shows a difference of reflection factors between gold and silver due to a difference of polarization directions (reference FIG. 4).
Usually, an angle of incidence of 30.degree. to 70.degree. is necessary in a laser scanning optical apparatus, and the difference of reflection factors between the two laser beams 6.sub.P and 6.sub.S is a problem as illustrated in FIG. 4. Such a difference of reflection factors occurs on a reflection surface of a rotating polygonal mirror formed by a general aluminum material as well as on a multi-layer reflection film having multiple layers of dielectrics. In other words, in an optical apparatus for a laser printer using two laser beams as shown in FIG. 3, the polarized beams of laser beams after they have been amalgamated are mutually orthogonal with each other, and when the laser beams are incident directly to the rotating polygonal mirror, the above-described difference of reflection factors occurs.